Electric discharge lamp



Aug. 28, 1956 J. N: BOWTELL ELECTRIC DISCHARGE LAMP Filed Aug. 14. 1951 0 b n e V n I James N; Bowtell yW A W HIS Attor ney United States Patent ELECTRIC DISCHARGE LAMP James Norman Bowtell, Wembley, England,

assignor to General Electric Company,

Claims priority, application Great Britain September 4, 50

2 Claims. (Cl. 313--213) This invention relates to electric discharge lamps of the type adapted to be operated so as to produce intense flashes of light of short duration. The invention also relates to a method of manufacturing such lamps.

Lamps of this type, which are hereinafter referred to as flash-lamps and which may be employed, for eX- ample, for various stroboscopic applications, in general comprise a glass envelope provided with metal electrodes and containing a filling of one or more of the rare gases. Usually, in the use of such lamps each flash is arranged to be produced by the discharge of a condenser through the lamp.

In some cases difiiculties have been experienced in obtaining reliable operation of lamps of the type referred to because the discharge sometimes fails to start, and it has been found that the reliability of operation maybe improved by activating the cathode. Usually the lamp discharges are unidirectional and one electrode only of the lamp is required to serve as cathode and need be activated, but if pulses in alternate directions are to be applied, then both electrodes of the lamp should be activated. However, known methods of activating the electrodes have not proved very successful in the cases of small lamps because of limitations of space available for manipulation of the activator: thus, in particular, in a very small lamp such as one in the form of a small U-shaped tube of only a few millimeters internal diameter, owing tothe small size of the electrodes and the limited space around them the application of an activator presents great practical difi'iculties.

It is one of the objects of the present invention to provide a flash lamp which incorporates an activator in a novel form particularly suitable for use in the case of very small flash lamps as aforesaid.

According to one aspect of the invention, an electric discharge lamp of the type specified contains, within the envelope, at small quantity of an alkali metal amalgam so arranged that it is either in contact with or is capable of being brought into contact with at least one electrode of the lamp.

According to another aspect of the invention, a method of manufacturing an electric discharge lamp of the type specified includes the steps of introducing into the envelope a small quantity of an alkali metal amalgam and causing the amalgam to come into contact with at least one electrode of the lamp.

Preferably the relative proportions of alkali metal and mercury present in the amalgam are such that the amalgam is in the liquid state at room temperature.

The preferred alkali metal for use inaccordance with the invention is potassium, although any alkali metal capable of readily forming an amalgam, which preferably is liquid at room temperature, may be used. When a potassium amalgam is used, the ratio of potassium to mercury therein is preferably about 1 to 20 by Weight: an

amalgam of this composition is still essentially liquid, for it is capable of flowing. Smaller proportions of potassium may, however, be used, for examplea proportion as low as by weight of the mercury having been foundto give useful results, and larger portions than may be used provided, usually, that the amalgam remains inthe liquid state.

I have found that flash lamps containing in the envelope an alkali metal amalgam, in accordance with the invention, give greatly improved reliability of operation, failures in starting being greatly reduced. This effect is apparently attributable to cathode activation by the alkali metal in the amalgam, although it is possible that the alkali metal also has some gettering action on trace impurities. The main function of the mercury is to provide a vehicle for introducing the requisite small amount of alkali metal into the lamp envelope, and the use of a liquid amalgam enables the alkali metal to be introduced into the envelope by a simple, convenient method to be described here inafters The lamp filling usually comprises one or more of the rare gases, especially xenon, argon, neon or krypton, at low pressure, although for some purposes flash lamps with high pressure fillings may be employed. In either case, the presence of a smallamount of mercury vapor in the gasfilling inoperation of the lamp is not objectionable. For example, the temperature attained by low pressure flash lamps in operation is usually of the order of 100 C., so that the vaporpressureof the mercury, present in the amalgam, is only about 0.5 to '1 mm. andthis has no deleterious effect onthedischarge.

The amount of alkali metal amalgam present inthelamp envelope is not critical, provided that sufiicient alkali metal is present to activateeach electrode which is to function as cathode. For example, a tubular lamp envelope of about 3 mm. diameter and 60 to mm. long, for unidirectionaloperation, i. e., having a single cathode, may contain between about mgms. and about300 mgms. of potassium amalgam in which theratio of potassium to mercury is 1 to 20, by weight. Similar amounts of this potassium amalgam are, however, also suitable for considerably larger lamps.

In the preferred method of manufacturinga flash lamp according to the invention, the electrodes are first sealed into the envelope, which is then evacuated, the gas filling and alkali metal amalgam are introduced into the envelope, the envelope is sealed, the amalgam is brought into contact with oneor each of the electrodes, as required, for a'suflicient length of time to ensure that an adequate amount of alkali metal is deposited on the surface of the electrode, and finally the lamp is subjected to an aging treatment. Although it is preferred, as stated above, to activate the electrode, by bringing the amalgam into contact therewith, beforethe aging treatment is carried out, theiactivation of the'electrodes may, if desired, be effected during the aging process.

In one specific method of manufacturing a lamp in accordance with the-invention for unidirectional operation, the lamp envelope, into which the electrodes have already been sealed, is connected to a pump for evacuation of theenvelope and to a supply of a rare gas or mixture of rare gases forproviding the filling, in the usual manner, and is also connected to a vessel which contains a globule of mercuryand into which alkali metal vapor is arranged to be passed. The lamp envelope is evacuated and baked in known manner, the application-ofheat to the mercury and alkali metal at this stage being'avoided. After the baking step, andwhile the vacuum is still maintained, alkali metal vapor is passed over the mercury,

which is preferablywarm, to form an amalgam which is liquid at room temperature. The lamp cathode is then bombarded, and the rare gas iilling is introduced into the envelope, in known manner. The whole assembly is then sealed, the globule of amalgam is transferred to the lamp envelope, for example by tilting the assembly so that the globule runs into the envelope, the lamp envelope is sealed off from the remainder of the assembly, and finally the lamp is aged in an appropriate manner, the globule of amalgam being brought into contact with the cathode either before or during the early stages of the aging process. If both electrodes are required to function as cathodes, this method is modified in that both electrodes are bombarded, and in that the amalgam is brought into contact with both electrodes successively.

The specific method described above may readily be adapted for, and in fact is especially suitable for, the manufacture of a plurality of such flash lamps simultaneously. For example, a number of lamp envelopes may be sealed, by means of side tubes, to the same manifold, which is connected to a vacuum pump and to a supply of one or more rare gases and to which is attached a tube containing a globule of mercury and connected in turn to another tube containing a pellet of an alkali metal, preferably potassium. The mercury and potassium are present in the proportions required in the amalgam, and in amounts such that the amalgam produced will be somewhat in excess of the total amount required to be introduced into the lamps, for example an excess of about 30% is suitable.

The lamp envelopes, sealed to the manifold as described, iabove, are evacuated and baked, and the evacuation is continued during the formation of the amalgam. To efiect the latter, the mercury is preferably warmed to remove occluded air and the alkali metal is heated sufficiently to cause it to vaporize and distill over the warm mercury, when rapid amalgamation of the two metals takes place. When the formation of the amalgam has been completed the lamp cathodes are bombarded and the envelopes filled with rare gas, as in the manufacture of a single lamp, and then the whole manifold is sealed off. The amalgam is distributed approximately evenly among the gas-filled envelopes by tipping the assembly so that a small portionof the globule of amalgam runs into each envelope in turn. Each individual envelope is then sealed off, and each lamp thus completed is tilted or inverted so that the globule of amalgam surrounds and adheres to the cathode, and is allowed to remain in this position for some time. If desired, both electrodes may be treated in this way. The lamps are then aged, for example by passing a small direct current through them for about one hour.

One specific kind of flash lamp in accordance with the invention, and a method of manufacturing lamps of this kind, will now be described, by way of example, in connection with the drawing wherein Fig. l is an elevation, on an enlarged scale, of a flash lamp comprising the invention, and Fig. 2 is a perspective view of equipment utilized in the manufacture of a number of tubes.

The flash lamp of the example is .a small low frequency stroboscopic lamp, for unidirectional operation, comprising an envelope in the form of a U-shaped tube 1 (Fig. 1) of 3 to 3.5 mm. internal diameter, into each end of which is sealed a tungsten electrode 2 of 1.5 mm. diameter and 3 mm. long, the arc length between the two electrodes being 60 mm. The envelope contains a gas filling consisting of a mixture of xenon and argon at pressures of 10 mm. and 1 mm. respectively, and also contains between about 100 mgms. and about 300 mgms. of a liquid potassium amalgam consisting of potassium and mercury in the approximate proportions of 1 to 20, respectively, by weight.

"In manufacturing lamps of the kind described in this example, the potassium amalgam may conveniently be introduced into a plurality of the tubes simultaneously. Thus, each one of a number, for example ten, of the tubes 1 with electrodes already sealed in is sealed, by means of a side tube '4 attached at the bend of the U, to a side tube 5 of a glass manifold 6 which is connected to a vacuum pump and to a supply of the requisite mixture of xenon and argon, and to which is also connected a U-tube 7 with a side arm 8. 2.5 gins. of mercury 9 are introduced into the U-tube, and a pellet of potassium :10 weighing approximately 110-440 mgms. is introduced into the side arm, the open end of which is then sealed. It may be noted that accurate weighing of the potassium, which owing to the highly reactive nature of the metal presents difficulties of handling, is not essential in this case, provided that the Weight of potassium used is as near as possible to, and preferably within, the range specified.

The manifold 6 and the attached lamp envelopes 1, U-tube 7 and side arm 8 are evacuated, the lamp envelopes are baked, without heating the mercury or potassium, and then, while the manifold is still connected to the pump, the mercury 9 is gently warmed, to remove air bubbles, by applying a small flame to the outside of the tube 7 containing it, and the flame is then applied to that part of the side arm 8 containing the potassium 10 until the latter has all distilled over into the U-tube 7, where it rapidly combines with the mercury to form the required liquid amalgam. The lamp envelopes 1 are then filled with neon to a pressure of 10 to 20 mm. and the cathodes are bombarded by passing a current of 25 milliamps through the lamps for a few seconds, the envelopes are again evacuated and allowed to cool, and the mixture of xenon and argon in the proportions of 10 to 1, respectively, by volume, is passed into the envelopes to the required pressure. The manifold 6 is then sealed, and the whole assembly is tilted to allow a small portion of the amalgam to run into each lamp envelope, the portions of amalgam thus introduced into the lamps each being between mgms. and 300 mgms. in weight. The individual lamp envelopes are then sealed off and detached from the manifold, and each lamp is then tilted in such a way that the globule of amalgam therein surrounds and adheres to the cathode for a suflicient length of time to ensure that a film of potassium is deposited on the cathode.

The lamps are then aged by passing a small current, for example 2.5 milliamps, through them for one hour.

The method of introducing the alkali metal amalgam simultaneously into a number of lamp envelopes, as described in the above example, is particularly advantageous in the manufacture of small flash lamps of the kind de' scribed in the example, since the difficulty of introducing an activator into small tubes is overcome by this method. However, the method may also conveniently be employed for introducing an alkali metal amalgam into a plurality of larger tubes.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric gaseous discharge flash lamp comprising a tubular envelope of a few millimeters diameter and containing an ionizable luminosity-producing inert gas filling productive of substantially the entire luminous output of the lamp under pulse energization, a pair of solid metal rod-like electrodes projecting a short distance into opposite ends of said envelope for an electric discharge therebetween, and an activating coating consisting solely of alkali metal on at least one of said electrodes and deposited thereon from a small quantity of liquid alkali metal amalgam in said envelope and arranged to contact said electrode, the quantity of said amalgam being insufficient to completely submerge the electrodes in any position of the lamp.

2. An electric gaseous discharge flash lamp comprising a tubular envelope of a few millimeters diameter and containing an ionizable luminosity-producing inert gas filling productive of substantially the entire luminous output of the lamp under pulse energization, a pair of solid metal rod-like electrodes projecting a short distance into opposite ends of said envelope for an electric discharge References Cited in the file of this patent therebetween, and an activating coating consisting solely UNITED STATES PATENTS of potassium metal on at least one of said electrodes and deposited thereon from a small quantity of liquid 1O8874O Stemmetz l 19 potassium amalgam in said envelope and arranged to 5 Beneaud contact said electrode, the quantity of said amalgam being Stone at 1937 insuflficient to completely submerge the electrodes in any 2166351 Germer July 1939 position of the lamp 2,228,327 Spanner Ian. 14, 1941 2,391,573 Herzog Dec, 25, 1945 

1. AN ELECTRIC GASEOUS DISCHARGE FLASH LAMP COMPRISING A TUBULAR ENVELOPE OF A FEW MILLIMETERS DIAMETER AND CONTAINING AN IONIZABLE LUMINOSITY-PRODUCING INERT GAS FILLING PRODUCTIVE OF SUBSTANTIALLY THE ENTIRE LUMINOUS OUTPUT OF THE LAMP UNDER PULSE ENERGIZATION, A PAIR OF SOLID METAL ROD-LIKE ELECTRODES PROJECTING A SHORT DISTANCE INTO OPPOSITE ENDS OF SAID ENVELOPE FOR AN ELECTRIC DISCHARGE THEREBETWEEN, AND AN ACTIVATING COATING CONSISTING SOLELY OF ALKALI METAL ON AT LEAST ONE OF SAID ELECTRODES AND DEPOSITED THEREON FROM A SMALL QUANTITY OF LIQUID ALKALI METAL AMALGAM IN SAID ENVELOPE AND ARRANGED TO CONTACT SAID ELECTRODE, THE QUANTITY OF SAID AMALGAM BEING INSUFFICIENT TO COMPLETELY SUBMERGE THE ELECTRODES IN ANY POSITION OF THE LAMP. 